1. Field of the Invention
The present invention relates to a method of measuring the orthogonality of a movement coordinate system of a stage for two-dimensionally moving a substrate mounted thereon in an exposure apparatus for manufacturing a semiconductor memory, a liquid crystal display, or the like.
2. Related Background Art
In a conventional method of aligning a two-dimensionally movable stage in an exposure apparatus, two plane mirrors are fixed to the stage such that their reflection surfaces are orthogonal to each other, light beams are irradiated onto the two plane mirrors, and the distances to the plane mirrors are measured on the basis of the reflected light beams, thereby performing alignment. In this technique, the two moving axes of the stage are determined by the two plane mirrors, respectively, and a movement coordinate system is defined by the two moving axes. Therefore, if the two plane mirrors have a mounting error (the reflection surfaces are not orthogonal to each other), the X- and Y-axes of the stage movement coordinate system, which are determined by the two plane mirrors, are not completely orthogonal to each other. In this state, when a pattern on a reticle is to be exposed on a photosensitive substrate (to be simply referred to as a substrate hereinafter) mounted on the stage by a so-called step and repeat method, the pattern array coordinate system formed on the substrate has poor orthogonality. For this reason, the exposed pattern is shifted from a desired position (designed target position), i.e., misalignment between the reticle and the substrate occurs.
Conventionally, to correct such alignment error of the stage, test exposure is performed in advance to measure the orthogonality of the stage movement coordinate system. The obtained orthogonality is set as a correction value for alignment of the substrate, thereby performing an exposure operation. In this test exposure, a test pattern is exposed on a substrate mounted on the stage in several shots along the X- and Y-axes of the stage such that, e.g., a cross-shaped shot array is obtained, as shown in FIG. 6. The substrate is then rotated by 90 degrees and mounted. Using one X-direction alignment mark and two Y-direction alignment marks, which are formed in the shots exposed in the first exposure operation, the substrate is aligned in the X, Y, and rotating directions. The test pattern is exposed again as in the first exposure operation.
Thereafter, shift amounts X.sub.A and X.sub.B of the X-direction measurement marks which are formed in the first shot (indicated by a broken line) and the second shot (indicated by a solid line), are measured at a plurality of positions separated from each other by a distance L. The orthogonality, .alpha., is approximately obtained in accordance with the following equation: EQU .alpha.=(X.sub.A -X.sub.B)/2L (rad)
The shift amounts X.sub.A and X.sub.B are defined to be positive when the second shot position is shifted from the first shot position in the -X direction. The orthogonality .alpha. is defined to be positive in the counterclockwise direction.
The stage orthogonality may change due to a change in ambient temperature, an impact upon movement of the stage, or the like. For this reason, the orthogonality need to be periodically measured to update the correction value of the apparatus.
However, in the orthogonality measurement by the conventional test exposure, two exposure operations and the operation of measuring the shift amounts of measurement marks are needed. That is, periodical orthogonality measurements and management require a lot of labor and time. For this reason, actually, test exposure is performed only upon setting up of the exposure apparatus to obtain an orthogonality correction value, and thereafter, no further orthogonality measurement is performed until an exposure failure occurs due to a change in orthogonality.